|
|
|
The Problem Two essential requirements for comfortable and healthy indoor environments are adequate ventilation and good humidity control. Unfortunately in humid climates, which includes much of the densely populated regions of the world, it is difficult to meet both these requirements without using a lot of electricity. The fundamental problem is that a conventional cooling coil (whether using chilled water or direct-expansion refrigerant) cannot effectively meet the latent loads from ventilation on very humid days. All conventional chillers and air conditioners are essentially sensible cooling devices that dehumidify by lowering air temperature below its dewpoint so that moisture condenses. These systems must run with a wet cooling coil and the air that leaves this coil must be close to saturation. The limitation of conventional chillers and direct-expansion (DX) air conditioners becomes evident when one tries to use them in an advanced HVAC system. Technologies such as displacement ventilation, chilled beams, and radiate panels can be part of a low-energy HVAC system that eliminates the fan energy used in a conventional system that recirculates large volumes of air. However, these advanced systems will not work with a conventional chiller or DX air conditioner that supplies relatively cold air (e.g., 50 to 55 F) that is saturated with moisture (i.e., 100% rh). What is required is a cooling system that supplies drier, but warmer air.
The Solution: A Liquid Desiccant Air Conditioner (LDAC) Liquid desiccants are solutions that have a high affinity for water vapor. This property is the key to creating cooling systems that dehumidify air without over-cooling. (Dr. Lowenstein's review paper presents a more detailed explain of the theory, operation and status of liquid desiccant air conditioners than can be included here.) Since the 1930s, liquid desiccants have been used in industrial dehumidifiers. The liquid desiccants used in these systems commonly are very strong solutions of the ionic salts lithium chloride and calcium chloride. These ionic salts have the attractive characteristic that the salt themselves have essential zero vapor pressure, and so vapors of the desiccant will not appear in the air supplied by the LDAC. However, zero vapor pressure comes with a price: as with seawater (a chemically related salt solution), solutions of lithium and calcium chloride are very corrosive. This corrosiveness requires that all wetted parts within the LDAC be protected and that no droplets of desiccant are entrained in the supply air. A desiccant has the ability to dry air without cooling because it forms a relatively strong bond with water molecules (i.e., a stronger bond than that between molecules in pure liquid water). Whereas the heat released when water condenses (i.e., the latent heat of condensation) is approximately 1,000 Btu/lb, more heat--typically an additional 50 to 100 Btu/lb--will be released when water vapor "condenses" into a liquid desiccant due to the stronger bonds between the molecules. At this point it is important to recognize that a desiccant's ability to dry air decreases as its temperature increases. If a 43% solution of lithium chloride at 80 F were to absorb an amount of water vapor that diluted it to 42%, its temperature would increase to 123 F (assuming the desiccant is not cooled). Whereas the desiccant initially could dry air to 23.2 grains, after increasing in temperature and becoming slightly more dilute, its ability to dry air increases dramatically to 107.6 grains. Most of this loss in drying potential is caused by the increase in temperature. One approach to limiting the impact of the heat released when the desiccant absorbs water vapor is to flow desiccant at a sufficiently high rate that its temperature rise is limited (i.e., in the preceding example, if the desiccant's concentration changed only 0.1 point, its temperature would rise only only be 4 F). This approach, which was first used in the liquid-desiccant systems of the 1930s and is still used in many LDACs today, requires that the liquid desiccant is first cooled before it is sprayed or dripped over a bed of porous contact media. The air that is to be dried then comes in direct contact with the liquid desiccant as the air is drawn through the porous bed. Typically, a ratio of the desiccant-to-air mass flow ratio that is on the order of one will limit the temperature rise of the desiccant. The preceding "high flow" liquid desiccant system has the following disadvantages: (1) a large volume of desiccant must be circulated requiring large pumps with relatively large power draws, (2) the air flowing through the highly flooded porous beds has a relatively high pressure drop which increases fan power, (3) a separate heat exchanger is required to cool the desiccant before it is delivered to the porous bed, and (4) the air will entrain droplets of desiccant as it flows through the highly flooded porous bed. This last disadvantage is particularly important because of the corrosiveness of the desiccant. Carryover of desiccant droplets can be eliminated by droplet filters, but at the expense of additional pressure drop. In 1994 AIL Research received a U.S. patent for an LDAC in which the flow of desiccant is more than an order of magnitude less than that used in flooded-bed systems. This low flow of desiccant was achieved by replacing the bed of porous contact media of the high-flow system with a plastic heat exchanger that continually cools the desiccant as it absorbed water vapor. The continuous cooling of the desiccant insures that the desiccant maintains its drying potential despite the heat released as it absorbs water.
The dilute desiccant that leaves the conditioner is pumped to the regenerator. The regenerator has the same configuration as the conditioner: a parallel-plate liquid-to-air heat exchanger. Again, very thin films of desiccant flow in wicks on the outer surfaces of the plates, and air flows in the gaps between the plates. For the regenerator, however, a hot heat transfer fluid flows within the plates. This hot fluid can be supplied by a gas-fired boiler, solar thermal collectors, recovered heat from an engine or fuel cell, or other energy source. As the temperature of the desiccant increases, water evaporates into an air stream that is separate from and about one-tenth the volume of the air that flows through the conditioner. This air stream is discharged outdoors. This regenerator is commonly referred to as a scavenging-air regenerator. The hot, concentrated desiccant that leaves the regenerator and the cool, dilute desiccant that flows to the regenerator exchange thermal energy in the interchange heat exchanger. This exchange increases the efficiency of the regenerator and decreases the cooling load on the conditioner. The efficiency of the regenerator can also be increased by adding an air-to-air heat exchanger to preheat the air that enters the regenerator using the warm, humid air that leaves it. (This air-to-air heat exchanger is not shown in the preceding figure.) In both the regenerator and conditioner, the flow rate of desiccant is so low that the falling films on the plates are contained completely within the thin wicks. The air velocity over the films is too low to entrain desiccant droplets. Since both the desiccant delivery to and collection from the plates are done without creating droplets, desiccant does not carryover and there is no need for droplet filters. How does a low-flow LDAC solve the "overcooling/reheat" dilemma? A low-flow LDAC that uses 43% lithium chloride will supply air at about a 20% relative humidity and a temperature that is 6 F to 8 F above the temperature of the cooling water supplied to the conditioner. On a typical humid summer day, an LDAC that processes outdoor air and is cooled with water from a cooling tower will come close to isothermally drying the air (i.e., it performs almost 100% latent cooling). This LDAC could meet the requirements of the earlier example (i.e., condition air from 86 dry-bulb and 78 F wet-bulb to 65 F and 50% rh) by directly performing 31.7 tons of latent cooling and then relying on a separate sensible cooling device for the remaining 11.5 tons of sensible cooling. In this application, the low-flow LDAC eliminates the 10.8 tons of overcooling. Of the remaining 43.2 tons of required cooling, only 11.5 tons has to be handled by a compressor-based refrigeration system, the remaining 31.7 tons being rejected to ambient by the cooling tower. Of course the latent load served by the LDAC and cooling tower has an energy price associated it: the thermal energy that is needed to regenerate the desiccant. The Implementation of Low-Flow Liquid-Desiccant Technology The advantages of a low-flow LDAC are most effectively captured by implementing the three core components--the conditioner, the regenerator and the interchange heat exchanger as plastic heat exchangers. A 6,000-cfm LDAC that integrates a water-cooled conditioner, a hot-water regenerator and a plastic interchange heat exchanger are described elsewhere on our website, as is our current work to include a high efficiency, gas-fired 1½-effect regenerator in the LDAC. |
Typical
supply air conditions for displacement ventilation are 65 F and 50%
relative humidity. This supply air has an absolute humidity of 45.5
As
shown in the neighboring figure, a "low flow" LDAC has three main
components: the conditioner, the regenerator and the interchange heat exchanger
(IHX).
The conditioner is a parallel-plate liquid-to-air heat exchanger. A
coolant, typically cooling tower water (but possibly water from a geothermal
well, lake or chilled water loop), flows within the plates and a very low flow of liquid desiccant flows
down the outer surfaces of the plates. Thin wicks on the plate surfaces
create uniform desiccant films. The air to be processed flows horizontally
through the gaps between the plates. As this humid air comes in contact
with the desiccant, water vapor is absorbed. The heat released by this
absorption is transferred to the coolant. The air leaves the conditioner
much drier, although its temperature may not significantly change.